Harmful substances such as NOx are contained in the exhaust gases of internal combustion engines. In order to reduce the emission of these harmful substances, it has been known that in an exhaust system of an internal combustion engine, there is arranged an occlusion reduction type NOx catalyst which occludes (including “absorbs” and “adsorbs”) the NOx in an exhaust gas in a state where the air fuel ratio of the exhaust gas is lean, and at the same time releases and reduces the NOx thus occluded in a state where the air fuel ratio of the exhaust gas is rich. In this technique, the purification capacity of the catalyst decreases as the amount of NOx occluded (including “absorbed” and “adsorbed”) increases, so by performing enrich control on the air fuel ratio of the exhaust gas, a reducing agent is supplied to the occlusion reduction type NOx catalyst, whereby the NOx occluded in the catalyst is released and reduced (hereinafter referred to as a “NOx reduction treatment”).
In addition, in order to eliminate SOx poisoning in which the SOx in an exhaust gas is occluded to an occlusion reduction type NOx catalyst thereby to lower the purification capacity thereof, the bed temperature of the occlusion reduction type NOx catalyst may be caused to rise, and at the same time a reducing agent may be supplied (hereinafter referred to as “SOx poisoning recovery processing”), and in this SOx poisoning recovery processing, too, the reducing agent is supplied to the occlusion reduction type NOx catalyst according to the enrich control of the air fuel ratio of the exhaust gas.
Also, particulate matters (PM) having carbon as a principal component thereof is contained in the exhaust gas of an internal combustion engine. In order to prevent the emission of these particulate matters to atmospheric air, there has been known a technique of arranging a particulate filter (hereinafter referred to as a “filter”) for collecting the particulate matters in an exhaust system of an internal combustion engine.
In addition, in the above-mentioned filter, there may be one in which an oxygen occlusion material such as ceria, alkaline earth metals, etc., and a catalyst precious metal such as Pt are supported thereon so as to provide an oxygen occlusion ability. Such a filter performs not only the function of collecting particulate matters, but also the function of occluding (including “adsorbing and absorbing”) the oxygen in an exhaust gas when the air fuel ratio of the exhaust gas is lean, and releasing the occluded oxygen to the exhaust gas when the air fuel ratio of the exhaust gas becomes rich. Then, the filter performs the oxidation purification of the CO and HC in the exhaust gas by the use of the oxygen thus released. Accordingly, it is possible to consume and purify the HC and CO in the exhaust gas of a rich atmosphere by means of the filter having this oxygen occlusion function (hereinafter also referred to as an OSC (oxygen storage capacity) function).
In the exhaust system of an internal combustion engine, there may be provided a plurality of exhaust gas purification devices such as the above-mentioned occlusion reduction type NOx catalyst and the filter having an OSC function, etc., that are arranged in series with one another. In such a case, it may be necessary to determine the degradation in function of the occlusion reduction type NOx catalyst and the filter with an OSC function.
In relation to the technique of determining the degradation of an exhaust gas purification device(s), there has been well known a technique in which the degradation diagnosis of a NOx trap catalyst, which is inserted in an exhaust passage for trapping NOx therein in an oxidizing atmosphere, is performed based on a period of time after the air fuel ratio of an exhaust gas in the upstream side of the catalyst has become rich until when the air fuel ratio thereof in the downstream side of the catalyst becomes rich during the control in which the NOx trapped is reduced and purified (see, for example, a first patent document).
In addition, there has also been known a technique in which a difference between the amounts of generated heat per unit flow rate of combustibles generated by the reactions of the combustibles and others in a catalyst casing of a catalytic converter is calculated from the temperatures of an exhaust gas at the upstream and downstream sides of a catalyst that oxidizes the combustibles in the exhaust gas of an engine and an atmospheric temperature, and when the number of determinations that the difference is equal to or less than a preset determination value becomes equal to or more than a prescribed number of times, it is determined that the catalyst is in a degraded state (see, for example, a second patent document).
In the above-mentioned conventional techniques, however, there has been a problem that it is difficult to determine the degradation of the function of each of a plurality of exhaust gas purification devices such as NOx occlusion reduction type catalysts, filters with an OSC function, etc., in a distinguished manner. In addition, in order to determine the degradation of the function of each of the plurality of exhaust gas purification devices in a distinguished manner, there has been a further problem of an increase in the device cost due to the provision of sensors one for each of the exhaust gas purification devices.